The present invention relates generally to chemical sensing apparatus and, more particularly, to apparatus for sensing hydrogen over an extended range of concentrations.
Providing chemical sensing apparatus, and especially apparatus for sensing hydrogen, having the capability to detect chemicals over an extended range of concentrations continues to present a technological challenge in spite of recent advance in the field of chemical sensing apparatus. While a wide variety of solid state chemical sensors have been developed, such sensors have generally been limited to detecting low concentrations of hydrogen. Exemplary of such solid state sensors include: metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) capacitors and field effect transistors (FET) as well as palladium gated diodes. A first MISFET sensor is described by Svensson et.al. in U.S. Pat. No. 4,058,368 wherein the metal electrode or gate is preferably palladium; although, Svennson et al. suggest that less sensitive devices could also be made with a nickel or platinum metal electrode. Improvements on such MISFET sensor are described by Sibbald et. al. in U.S. Pat. No. 4,931,851, wherein the metal electrode or gate comprises, either platinum or palladium and SiO.sub.2 mixed with or deposited on an exposed surface of the metal electrode. Whereas, Raul in U.S. Pat. No. 4,892,834, describes the use of sandwiched layers of metal oxides, pure catalytic metals, insulated oxides and semiconducting material in forming his solid state chemical sensor. More recently, Hughes et al. describe a palladium silver alloy diode for sensing hydrogen in "Thin-film palladium and silver alloys and layers for metal-insulator-semiconductor sensor" J. App. Phys. 63(1), 1987 pgs. 1074-1083. Alternatively, McNally in U.S. Pat. No. 4,313,907 describes a chemical sensor employing a wheatstone bridge wherein one of the resistive legs comprises a platinum wire coated with a mixture of palladium, palladium oxide and nickel oxide. While Johnson et. al. in U.S. Pat. No. 4,953,387 describes an alternative resistive semiconductor device.
In spite of such advances, there remains a need for chemical sensing apparatus capable of detecting a dynamic range of hydrogen concentrations over at least six orders of magnitude (i.e. 10.sup.6). Moreover, there remains a need for a chemical sensing apparatus which responds rapidly and reversibly to changes in concentrations at room temperatures while resisting the poisoning effects of materials such as H.sub.2 S.